A small engine serving as a power source is used in a portable working machine or a generator that is carried and used by an operator, for example, a bush cutter, an air blower, a chain saw, a power cutter, or the like. FIG. 8 is an exterior view of a bush cutter serving as an example of an engine-driven working machine 501 in the related art. The engine-driven working machine 501 on which a small 2-cycle engine is mounted rotates a rotating blade 512 installed at the other end of a main pipe 504 due to a driving shaft (not shown) passing through the main pipe 504 on the pipe and the driving shaft being rotated using an engine 510 installed at one end of the main pipe. A scattering prevention cover 513 configured to prevent scattering of reaped grass is installed in the vicinity of the rotating blade 512. The engine-driven working machine 501 is carried by a shoulder belt or the like (not shown), and a handle 508 having a U shape when seen in a front view and configured to be operated by an operator is attached to the vicinity of a longitudinal central portion of the main pipe 504. The rotational speed of the engine is controlled by an operator using a throttle lever 507 attached to a grip portion 509. An operation of the throttle lever 507 is transmitted to a carburetor 25 of the engine 510 by a throttle wire 523.
Since a high temperature exhaust gas (hereinafter, simply referred to as “an exhaust gas”) after combustion is discharged according to the reciprocal movement of a piston and explosion due to combustion of a fuel-air mixture in a cylinder of the engine, the surroundings of a muffler are covered with a muffler cover 540 such that an operator does not directly come into contact with the heated muffler. In addition, in recent years, various ideas have been proposed to prevent a surface temperature of the muffler cover from being excessively increased or to decrease a temperature of an exhaust gas discharged outside of the muffler cover as much as possible. For example, in order to diffuse the exhaust gas discharged from the muffler, a technology of achieving a decrease in exhaust gas temperature by forming a plurality of small holes in a discharge port of the muffler or causing some of engine-cooling air to flow around a muffler chamber as disclosed in Patent Literature 1 may be provided.